RESUMEN
Three dimension atom probe analysis of semiconductor materials requires the ability to bring high electric field at the specimen apex to remove atoms. It is shown that, if voltage pulses are used to evaporate doped silicon, the resistivity of the material has to be lower than about 10(2) Omega cm. To overcome this problem, voltage pulses have been replaced by femtosecond laser pulses. The laser pulses give rise to field evaporation by two processes. Both thermal and optical field evaporation have been observed. Thermal evaporation takes place at high laser intensities and with short wavelengths while the evaporation is assisted by the rectification of the optical field for lower intensities and in the infrared domain. Using the optical field evaporation, reproducible and good analyses in term of spatial and mass resolutions could be conducted.
RESUMEN
Lead chalcogenide (PbS, PbSe, and PbTe) nanocrystals were synthesized by sequential implantation of Pb and one of the chalcogen species into pure silica. The implantation energy and fluence were chosen so that the implantation profiles practically overlap at a depth approximately 150 nm with a maximum concentration of about 0.3 atom %. Annealing for 1-8 h at 850-900 degrees C triggers nanocrystal growth, which is monitored by high-resolution (HRTEM) and conventional transmission electron microscopy (TEM), secondary-ion mass spectrometry (SIMS), and Rutherford backscattering spectrometry (RBS). Striking differences are found in the depth distributions and microstructures of the resulting nanocrystals. We show that the differing chemical interactions of Pb and chalcogens (between each other and with silica) play a crucial role in chalcogenide nucleation and growth. Using available information on chalcogen redox states in silicate glass, we propose a nonclassical nucleation and growth mechanism consistent with our experimental results. The complex chemistry involved at the microscopic level is shown to impair control over the nanocrystal size distribution. Finally, PbS nanocrystal-doped silica is shown to emit intense photoluminescence (PL) in the 1.5-2 microm wavelength range, an effect that we relate to the above nucleation and growth scheme.
RESUMEN
We demonstrate that room temperature MeV ion irradiation of a glass containing copper oxide initiates nucleation of pure Cu clusters via the inelastic "electronic" component of the ion energy loss, when the latter is above a threshold value. The clusters grow under subsequent thermal annealing, following Lifshitz-Slyozov-Wagner kinetics. The decoupling of nucleation and growth is analogous to that occurring in the photographic process. It allows total control over the cluster density, average size, and size distribution.
RESUMEN
Magnetization reversal in a periodic magnetic field is studied on an ultrathin, ultrasoft ferromagnetic Pt/Co(0.5 nm)/Pt trilayer exhibiting weak random domain wall (DW) pinning. The DW motion is imaged by polar magneto-optic Kerr effect microscopy and monitored by superconducting quantum interference device susceptometry. In close agreement with model predictions, the complex linear ac susceptibility corroborates the dynamic DW modes segmental relaxation, creep, slide, and switching.
RESUMEN
As a magnetic domain wall propagates under small fields through a random potential, it roughens as a result of weak collective pinning, known as creep. Using Kerr microscopy, we report experimental evidence of a surprising deroughening of wall pairs in the creep regime, in a 0.5 nm thick Co layer with perpendicular anisotropy. A bound state is found in cases where two rough domains nucleated far away from one another and first growing under the action of a magnetic field eventually do not merge. The two domains remain separated by a strip of unreversed magnetization, characterized by flat edges and stabilized by dipolar fields. A creep theory that includes dipolar interactions between domains successfully accounts for (i) the domain wall deroughening as the width of the strip decreases and (ii) the quasistatic and dynamic field dependence of the strip width s.
RESUMEN
We show how, combining He ion irradiation and thermal mobility below 600 K, the transformation from chemical disorder to order in thin films of an intermetallic ferromagnet (FePd) may be triggered and controlled. Kinetic Monte Carlo simulations show that the initial directional short range order determines the transformation. Magnetic ordering perpendicular to the film plane was achieved, promoting the initially weak magnetic anisotropy to the highest values known for FePd films. Applications to ultrahigh density magnetic recording are suggested.
RESUMEN
By ion irradiation through a lithographically made resist mask, the magnetic properties of cobalt-platinum simple sandwiches and multilayers were patterned without affecting their roughness and optical properties. This was demonstrated on arrays of 1-micrometer lines by near- and far-field magnetooptical microscopy. The coercive force and magnetic anisotropy of the irradiated regions can be accurately controlled by the irradiation fluence. If combined with high-resolution lithography, this technique holds promise for ultrahigh-density magnetic recording applications.